As a result of screening 46 antibodies directed against defined regions of rhodopsin, we identified a monoclonal antibody, which also recognizes opiate receptors from NG108-15 neuroblastoma x glioma hybrid cells. A peptide corresponding to the epitope of the antibody inhibits precipitation of opiate receptors by the antibody. A peptide from the homologous region of the porcine brain muscarinic receptor not only blocks the antibody, but also activates a number of G-proteins both in membranes and in solutions of the purified proteins. Homologous peptide fragments of other receptors also activate the appropriate G-proteins. These experiments suggest that we have identified an important region of the signal transmitting domain of G-protein coupled receptors. In related experiments, we have shown that the alkylating GTP analogue fluorosulfonylbenzoylguanosine (FSBG) can be used as a reagent to inactivate G-proteins in natural membranes without affecting receptor numbers of effector enzymes directly. Thus, FSBG treatment of NG108-15 membranes prevents bradykinin stimulation of phospholipase C activity without changing receptor numbers of the intrinsic or calcium activated enzyme activity. Bradykinin sensitivity can be restored to the treated membranes by addition of purified G-proteins.